This invention relates to energy detectors, and more particularly to such detectors having a weak primary response to incident energy pulses, followed by a stronger secondary response, such as pyroelectric infrared detectors.
Pyroelectric detectors are known, and described in U.S. Pat. Nos. 4,060,729, 3,773,564 and 4,117,328, for example.
As known in the art, such devices typically provide an output voltage which is proportional to the time rate of change of the temperature thereof. While such rate change responses are useful in detecting the existence of a pulse of incident energy, it is often desirable to detect the actual temporal waveshape of the incident energy pulse. Such temporal detection enables more detailed investigation of high frequency information in the incident energy.
For example, in the field of laser diagnostics, the determination of a laser's function is often made by observation of its output waveform. Thus, measurement of actual output intensity and its variation as a function of time is required, rather than merely detecting the presence of an output pulse. Other applications in which pulse-code modulated signals are detected similarly require observation of actual signal waveforms rather than detection of the existence of a signal or its variation.
Further, it is desired to increase the detectivity, or sensitivity, of energy detectors for reproducing substantially the exact shape of an incident pulse of energy.
Such responses are particularly desirable for high frequency pulses, for example pulses having sub-picosecond durations. Because of the slow response and poor detectivity of existing pyroelectric devices, these detectors are not useful in applications as above described.
However, pyroelectric devices are now known which include both a primary and a secondary response. The "primary pyroelectricity" response is virtually simultaneous with the impinging energy pulse (such as infrared radiation), having a 10.sup.-13 second coupling time, for example. The wide band response available under the primary pyroelectric effect is, however, quite weak in comparison with the secondary pyroelectric response. It is accordingly the secondary response, occurring substantially later than the primary response, which is typically used in commercial utilizations of pyroelectric devices. Stotlar et al, in 1979 IEEE Symposium on Applications of Ferroelectrics, Symposium Digest, page 95, June 1979, describe the use of the primary pyroelectric response for detection of 10.6 micron laser pulses. Such applications of primary pyroelectric properties have thus far been limited to high energy pulses, however, because of the weak primary responses. Experimentation with the primary pyroelectric response has been limited to real time detection of high energy pulses. Application to low energy incident radiation has not been practical.
While the prior art includes a number of circuits for reducing noise in detected radiation, as illustrated by Cohn 4,081,679, Zeldman 3,723,737, and Douglas 3,855,864, the prior art has failed to provide a device for detection of weak signals using a primary pyroelectric effect.
In a more general sense, the prior art fails to provide either a method or apparatus for enabling reliable use of a primary response of an energy detector which also possesses a secondary response, where the primary response provides an accurate and rapid, but weak, response to incident energy, while the secondary response is stronger, but is delayed and has a lesser accuracy than the primary response. Thus, particularly with respect to pyroelectric detecting devices, the primary response, although it more accurately reflects the temporal waveshape of the incident radiation, is essentially not usable, particularly for detection of low power incident radiation.